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How can I calculate the C.G height for an Aircraft?
before I continue I would like to thank you guys beforehand for the help. I am new to this forum and in aviation in general. I am helping my professor to obtain the C.G coordinates in x, y , z for a simulation of a Cessna 172 (we are using Tornado for Matlab). From what Ive read, the C.G in aviation is mostly obtained through the longitudinal axis (x), I assume that the y is centered in most cases, but I am wondering, how can I obtain the height of the C.G, can I use the same method as in the longitudinal axis?
I was also wondering, why is it that for the calculation of the C.G you take into account things such as fuel, passengers, luggage and so on, but not other things like the motor, propeller, fuselage, landing gear, etc.?
Sorry if my questions are a little stupid or obvious. Thank you for your help!

Interesting question!  I have seen questions on lateral CG before, but never vertical.  However, I don't think there is any real practical value in this exercise since the vertical axis of the CG doesn't have any real effect on the flight characteristics.  At least none that I have ever encountered or even mulled over theoretically...

@MichaelHall Some models of the Boeing 767 have vertical c.g. limitations expressed as maximum vertical moments. If you want to see an example, go to 747.terryliittschwager.com. Dismiss the initial pop up window, then go to the SERVER box and under the Loads click on the second test file from the top, labeled 2222-01-07_TEST_B767cfgA_N760XX. A window for the load will come up with a problem notification. Dismiss that and then scroll down to the first envelope depicted graphically and you'll see it. It's entitled GROSS WEIGHT LIMITATIONS VERSUS CLIM AND DECK LOADING, VERTICAL CG UP TO 42".

Vertical CG may not vary much for pax and luggage for a 172, but for a "from scratch" design it is needed to determine side force torque from area above and below it.  The builders of the 172 did a terrific job, and, judging by its reluctance to roll too much in a side slip, it would be near the midpoint of areas top and bottom, somewhere near the pilot's sitting place.

No that's a really good question. Weight and balance calculations for flying are only concerned with the location and value of the variable weights - fuel, pax, baggage. The C of G of the "empty" aircraft with all those components is considered a single unchanging value for the purpose of establishing a starting point before loading, and is established just by weighing the airplane while empty.  The weight of the components is a factor in where the empty C of G will end up, but once established the entire "empty" machine is treated as a whole unit for W&B purposes.
Vertical C of G is not normally provided with A/C weight and balance data.  Although the location is not too big deal with most types of aircraft, for gyrocopters it's quite important because they need to have the engine's thrust line as close as possible to the center of mass for safety reasons (if you unload the rotor on a gyro by pushing over or from turbulence, and the thrust line is significantly above the vertical C of G, the thrust line tries to spin the mass of the machine like a pinwheel about the C of G and the gyro tumbles, with fatal results; the tall and spindly look you see on small gyros today is driven by the need to mount the engine/prop lower to get the thrust line close to the vertical C of G).
On a gyro, the procedure is to hang the loaded ready-to-fly-state machine by the rotor hub, establish a vertical reference line below the hang point, then hang it by another point at the same level that is offset forward or aft using a special "2 hang point" rotor hub fixture, or alternatively if it'll work you can tip the machine back on its main gear until it balances, and establish a vertical reference line extending up from the gear contact point.  Where the line extending down from the rotor hub, and the line extending up from the gear balance point (or the two downward lines if you had a dual hang point fixture) intersect, is the vertical C of G.
To figure this out on a 172 you would need to do something pretty radical, like support it along the fuselage at one point using a suitable resting fixture just forward of the main gear but aft of the C of G, and tip it back until it balances and establish a vertical reference through the balance point, then move the fuselage rest forward to ahead of the C of G and tip the plane nose down to establish a new balance point and vertical reference.  Then where the vertical axis of each balance point intersects, there's your vertical C of G.  Of course you'd want to do this while loaded with people, fuel and bags and you'd probably have to pay them or offer them free beer, after you've found an airplane to use, built a fixture, and all that.  You could also probably do it by weighing on scales with the nose propped up high on a block so it points up at an angle, then reweighing with the mains propped up high on two blocks so it points down on an angle, then do some clever geometric calculations to work it out.
Or, if you are as lazy and cheap as me you could call Cessna's Tech Support organization and tell them what you are doing and what you need, and somebody somewhere should be able to find the data (hopefully) without charging you for it.

Thank you for the reply John and the others that commented on my question! The problem is that I dont have access to the physical aircraft. I wish I could now since what you are proposing seems like an interesting way to obtain the CGH. I also find it interesting that the CGH is not as relevant in most aircraft.  Thank you very much! I will start reading more about aviation it is really interesting!

The vertical CG is usually not that far from the engine's axis in most airplanes except amphibians with engines on pylons above and such.  If you eyeball a 172 and consider the volume of the wings and fuselage, you could guesstimate that the assembly's vert CG would be somewhere around the lower half of the side window.  The engine's center of mass is roughly about the same level; the gear would lower the CG a bit.  I will bet that if you get any results from Cessna it will turn out to be not too far above or below the lower edge of the window.  Be interesting to find out.

You can calculate it the same way they calculate the vertical CG on astronauts in space suits. First calculate CG with the aircraft level. Then raise the nose of the aircraft so that the deck angle increases to exactly 20 degrees and re-measure CG.
Now re-level the aircraft and plumb the second CG position to the ground, then plot a line 20 degrees forward of perpendicular from that point. Where that line intersects the CG taken in the first (level) measurement is the aircraft's vertical CG (CG height).


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What do left handed F-16 pilots do about the right side stick?
I was just watching a video about the Thunderbirds and Blue Angels doing their city fly overs and realized that the F-16 has a right hand side stick, which begs the question "what do left handed F-16 pilots do about it?" Adapt? Not get selected for that aircraft? These and other dumb questions for you

This has been asked before, search...

Any single-seat aircraft only has the throttle(s) on the left; none have them on both the left and the right.  All airliners have throttles only in the center.  This is an "every aircraft" issue, not an "F-16" issue.

It does not truly matter what hand the “stick” is in. It matters how you were trained. It matters to what you have become accustomed.
Case in point...
In general, the pilot in command of a side-by-side pilot aircraft sits in the left seat. This is the seat in which most people begin their initial pilot training. You will typically control the control yoke with your left hand while manipulating the throttle, mixture, propeller control, radios and avionics with your right hand. When you advance to being a Certificated Flight Instructor, you will move to the right seat. In the right seat, you will you will control the control yoke with your right hand while your left hand does everything else. If you go on to become a captain in an airline, you will move back to the left seat.
If the same pilot starts training in a helicopter, the pilot in command seat is typically on the right side. You would reverse again which hand controls what.
Your training for the particular aircraft and position you fill will dictate how you operate. Not with which hand you are more dominant. Both right-handed and left-handed pilots fly their respective aircraft the same way. They train for it.

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Do pilots typically mix roll/yaw when piloting various planes?
When flying on GeoFS, a popular flight simulator online, one of the default settings is to mix roll and yaw. Of course, it isn't real life, so I think it might differ from flying in the real world.
Do professional pilots mix roll and yaw? Is it possible that pilots have different settings for different types of planes, such as fighter jets and commercial jets?

What do you mean 'mix' roll and yaw?

Meaning a combination of roll and yaw when the control stick is moved to the left/right.

Does your “mix roll and yaw” setting eliminate the need to provide rudder inputs? If so, this is something a pilot does to coordinate the aircrafts turn. Some, more advanced aircraft have yaw dampers for cruise flight. But, many sim programs take into account that their users might not have rudder pedals.

That's correct. When mixing roll/yaw on the simulator, you can just move your mouse left to bank and yaw simultaneously. You can still control the rudder manually when you disable mix roll/yaw.

Related: Do pilots use the pedals in flight on planes with a sidestick (Airbus)?

@SovereignInquiry, most airplanes do not have a mechanism to coordinate rudder and aileron input, it's done manually.

Pilots typically make turns in the air by banking the aircraft. The act of banking the aircraft is done by moving ailerons. Since moving one aileron up and the other aileron down produces a roll in the direction of the up aileron, the aircraft will bank (and presumably turn) in the direction of the up aileron.
Induced drag is a byproduct of lift. By putting one aileron up and one aileron down, you are producing more lift on the down aileron side and less lift on the up aileron side. Therefore you are also producing a lot more drag on the down aileron side than you are on the up aileron side. This causes adverse yaw in the direction of the down aileron. So, while the bank is trying to fly the aircraft in one direction, the induced drag is trying to yaw the aircraft in the opposite direction. This is one type of adverse yaw.
While turning is done by banking through rolling instead of simply yawing, yawing must still be done. The main job of the rudder is to combat adverse yaw. Pilots are constantly making judicious use of the rudders to keep the flight of the aircraft coordinated and streamlined. Changes in pitch, roll, and airspeed can all cause adverse yaw.
Some more modern planes will do this to some degree for the pilot through electronics. The mechanism for technology to accomplish this is called the yaw damper. This, however, does not completely eliminate the need for rudder inputs like your “mix” setting does.

If your plane has a yaw damper system, the only time you'll ever need to touch the pedals is on takeoff/landing, and if an engine quits.  The rest of the time it's feet on the floor and the YD takes care of everything else.  When I learned to fly the RJ, I had to take my feet off the pedals once airborne to stop unconsciously squeezing bits of rudder every time I applied aileron when hand flying.

@JohnK - Thanks John. That was my understanding of a yaw damper as well. In the case of some PC sims, you can eliminate the need to use rudder pedals entirely. Even on takeoff and landing. Not everyone jumps into the deep end of the pool, expense-wise.

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Why does the pilot do this when taking off?
In the past (before the pandemic, of course), when riding commercial jets, I noticed that sometimes, the pilots would bank to port, then to starboard, and repeat that multiple times, while climbing after takeoff. I was wondering if there was a reason they did that. Maybe they were just turning? Then why bank back to the other side? Perhaps they did it because there's nothing illegal with banking while you're climbing? But that might generate lift in a direction not quite perpendicular to the horizontal plane. I'd think that they would do it for a reason. Perhaps no one is sure of what I'm talking about. Any help?

How much banking are we talking about? And for how long did the plane remain banked?

Well...I'm pretty sure the plane continued travelling in a straight direction. The banking was most likely 45 degrees or less. And the plane remained banked for around 10 (?) seconds or a bit less.

Yes, pilots turn the aircraft, and make heading corrections, both of which require some banking.  Looking out the side window at a long wing, even minor course corrections may seem larger than they really are.

@Sovereign Inquiry: 45 degrees is a pretty major bank angle.  I'd suspect your commercial flights bank much less.  As for why they bank, unless the runway is pointing directly at the destination, they pretty well have to turn.  And then there's noise abatement, traffic avoidance, and so on.  As for banking back and forth, I suspect much of that is basically a perceptual illusion.  The first bank starts a turn, then when you reach the desired heading, you "un-bank" to straighten out.  But if you don't have a good view of the horizon, or instruments, it feels like a bank the other way.

Ahh. And you're probably right about the bank angle. You could make your comment an answer...

@jamesqf Please make that an answer so we can upvote.

@StephenS: I hesitate to make something an answer when I'm basically just combining my own experience (in small planes) with a bit of logic and a bunch of guessing.  Going by my experience with other sites, if I do, someone is sure to ask for sources :-(

Not sure why I'm receiving downvotes...

I will try editing.

I think it's necessary for us to know how you knew that the airplane was banking. Was it because you looked outside and saw the horizon move up and down, or because you could feel it? The answer to your question is going to be very different in those two cases.

When a commercial airliner takes off, they are vectored (directed) by ATC along or to a certain course. Or, they are given a specific published, codified procedure to follow. This provides the aircraft proper obstacle and traffic avoidance and clearance. It might also provide for avoiding sensitive areas like restricted airspace, environmentally and noise sensitive areas. You can view published procedures for your airport on Sky Vector
When ATC or the published procedure requires the aircraft to change direction, the aircraft has to bank to turn. When an airliner turns, they typically bank no more than 30° unless there is an emergency. The 45° bank you mentioned would be avoided due to discomfort for most passengers. Bank angle exponentially increases G-load. A 45° bank would increase G-loads by almost 50%. And some passengers would get the sensation that the wings were pointing perpendicular to the ground.
S-turns (one turn immediately followed by another turn in the opposite direction), while not illegal, are not normal in an airliner. And, it would not make sense to do them on takeoff nor climb-out.
I would suggest that you were suffering from a vestibular illusion called Somatogyral Illusions. A mild form of this is usually termed “The Leans”. A more severe form of this is termed “Graveyard Spins” and “Graveyard Spirals”. You can combat and overcome this illusion by sitting with your head straight up and as still as possible. Abrupt head movements will greatly aggravate this condition. Being able to see the horizon out of the front windowsheild also helps. Unfortunately, you do not have this view.

So in other words, commercial jets basically never bank for no reason. And I think the Somatogyral Illusions aren't serious. jamesqf described it perfectly: "As for banking back and forth, I suspect much of that is basically a perceptual illusion. The first bank starts a turn, then when you reach the desired heading, you "un-bank" to straighten out."

@SovereignInquiry - If they are banking, there is probably a reason.

As far as the illusion, you would be surprised. It is very easy (for me) to tell when an aircraft is in level flight. It is very easy to tell when an airplane is banking. It is also very easy to tell when it is unbanking. When you do not have a view of the horizon or flight instruments as a reference, it is harder to tell the degree or magnitude of a bank or unbank. When an aircraft banks slow enough, remains banked long enough, or unbanks too rapidly, you could have the sensation of a bank in the opposite direction when the aircraft returns to straight and level.

Also, many maneuvers like turns and descents can be done in such a way as to simulate one positive G-load straight down relative to your seated position. In other words, you may not even feel the turn, bank, nor descent. The combination of these has caused many accidents. JFK, Jr being a very notable one. That is why pilots train to prevent, ignore, and overcome spatial disorientation.

Maybe remaining banked for an extended period of time is the explanation.

@SovereignInquiry They will usually be doing turns at standard rate, which means a 180° turn would take one minute. Without any outside reference or instruments, that’s easily long enough for your body to trick itself into thinking the plane is level again, so you feel a “turn” the other way when they straighten back out.


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Landing at 30,000 ft
Excluding how a runway would exist at 30,000 ft, could landing a plane at that altitude be done? What would it take to do so?

Very Basic Wind rose for Mount Everest
Weather Phenomena That Could Impact Aviation Operations:
High Density altitude (1/3) density altitude of sea level. Planes capable of a ceiling of 35000 ft required. Ground effect deceleration landing required.  Strong crosswinds and updrafts present year-round. Build from ice, the runway is 3 to 5 times longer to compensate for Density Altitude, wind, and ice. 10 percent chance of rain year-round. In the middle of June to September the monsoon season produces heavy precipitation. The temperature ranges from -2º F to -76º F. Banner clouds often present. The Subtropical jet stream maybe present with winds exceeding 80mph. If the Polar jet stream merges with the sub-tropical jet stream wind could exceeded 120mph. Supplemental oxygen maybe needed. Presume landing is done in head-wind only.

Landing is less of a problem. All it needs is a very long runway, tires strong enough for the higher speeds and a beefed-up landing gear for the higher sink speed. Taking off again from this runway will be the really hard part!

Does this answer your question? What would be the challenges to taking off and landing a typical passenger jet at FL300?

@PeterKämpf what about flap speed?

@Abdullah The flap speed limits are IAS limits and you would land at the same IAS. It's just that this IAS would correspond to a much higher TAS at these altitudes.


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What happens when a "ground effect" plane approaches the sound barrier?
Does the shock wave inhibit or contribute to lift? What is the ideal speed to fly near or on the speed of sound while using the ground effect?

very complicated question. the proximity of the ground means that the shock wave will bounce off the ground and come back at the underside of the wing and strongly interfere with airflow there.

This is a problem they must have solved for supersonic cars

To add to the complexity, in proximity to a wall, the shocks don't even reflect nicely like a tennis ball, you have very complex shock triple-points (not to be confused with phase triple-points) formed near the walls. This has very drastic effects on the boundary layer as well. There's lots of interesting CFD pics of landspeed record cars, like here: https://info.pointwise.com/webinar-attempting-break-land-speed-record


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Are the question pools for written exams publicly available?
Can you download the question pool with answers for the FAA knowledge exams?

@757toga so you can download the 2008 pool?


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What was this strange orange and grey plane I saw today?
Sitting out in the backyard today I heard a plane that seemed louder than usual, I turned and saw it immediately, it appeared to be flying quite low compared to the many I’ve watched fly over us in the past. It was also an odd colouring I’d never seen. Orange and dark grey. I didn’t see any text, but it was large like a commercial plane. I live in Northern Alberta, in Grande Prairie, where we really only have the same two companies with the same aircrafts that service the area. I also couldn’t see it on Flight Radar 24 when I immediately went to check what it was.
Any one have any ideas? I couldn’t even find an airline with that colour scheme. And it was gone too fast for me to get a photo despite having a very clear view.

"Louder than usual" and the colors aren't much to go on.  Prop or jet?  Single, or multi-engine?  Swept wing or straight?  Does it look similar to anything you've seen before?  Please help us by describing in a bit more detail.  (although maybe someone from that province will get it based on color... it could be a military trainer of some sort)

My first wild guess would be aerial firefighting... do you have ongoing wildfires in your region?

My only guess is a Newfoundland's Canadair CL-415:

Probably helping your province's firefighters.

This was pretty much what I was imagining as I typed my comment last night…. Good guess, let’s hope the OP circles back or this will get closed.

Just to triangulate, Alberta has four CL-215s, this pic shows three in yellow-and-orange. Quebec's 215s are blue and orange/yellow. cc @MichaelHall

and while Alberta Wildfire mostly talks about the helos they are using right now, this picture on an Aug 11 tweet shows the tail of a plane, so they're obviously using fixed wing on recent fires as well.

I know that Jet star (an Australian domestic and international flight company) has colours of grey and orange, and many of their planes are grey and orange, it could've been them.

Jet Star doesn't fly to Canada.

Jet Star is – as you said – an Australian operator. Their flights do show up on FR24 and it is highly unlikely that they operated in Canada.

Your answer could be improved with additional supporting information. Please [edit] to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center.


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Crossing extended runway centerline while intercepting LOC
I know it shouldn't happen but in case it happens for whatever reason, what is correct procedure to save the day. I mean the case that the aircraft is turning but it gets to the wrong side by, let's say, one, maybe two deg, so the pilot or autopilot has to make the correction and "fly back" to LOC.
I guess the ATC won't be happy, due to separation, protected areas of parallel runway or so.
But how to solve that? Can the aircraft fly back to LOC on its own, still asume it is within LOC deviation scale and we can see the diamond? Should they inform ATC that they will be on the wrong side for a while? Do they have to be vectored back to ILS again? I ask specificaly for this because I was told so but it seems to be quite ridiculous to issue a new vectoring for few seconds because before it will happen the aircraft will be already turning to LOC and it will be useless.

As a controller—if I issue an egregiously late turn, I will take responsibility and either issue a vector back toward the LOC or a re-sequence if necessary. If the turn is only slightly late, I expect the pilot to correct silently.

There was an issue in Toronto some years ago where some sort of Loc antenna issue would cause Air Can Jazz CRJs to start to capture, then spontaneously reverse and start to turn away. The pilot would have to go into HDG mode to correct and recapture, or if too far off, disconnect A/P and hand fly to keep in lateral limits. I don't remember what the actual root cause was.

@randomhead As I said, that turn was a bit late. Or I would call it a bit late, still visible on instruments where the LOC is, known deviation about 1 degree or 1,5 degree. So on ILS with final apch crs 180° the aircraft would be eg. 181-182° on 11 NM from RWY. Would you consider this as too much? The AC was turning back to LOC on its own.

I would expect the pilot (or autopilot) to silently intercept the localizer in that case, and probably even up to 5-10° overshoot. But it's a gray area; is the a/c 5° too far or 10°? Is the a/c type an F18, B752, LJ75, or P28A? Is it a student pilot training flight or commercial passenger flight? Are they 3 miles from FAF or 13?

@randomhead It is something like B737/A320, 2-3 NM from FAF and no more than 3°. I know it is grey area. i was curious about some real life opinions. Thanks a lot.

If you are instructed to intercept the localizer and/or cleared for the approach, that means you need to stay established on the localizer, i.e. keep the needle at leas than full deflection.
It doesn’t matter whether the needle is deflected left or right.  As long as it doesn’t hit either peg, you are still established and can maneuver as needed to get (and keep) the needle centered.
If you do hit the peg on either side, that’s a missed approach; tell ATC immediately.  Depending on where you are and other traffic, they may vector you back on or take you out and back for another try.
There are special rules for parallel approaches, but you should not be getting (or accepting) those without the proper training and authorization.


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Speed reductions prior to reaching a holding fix
The instrument ACS states that speed reductions for a hold should be made “3 minutes or LESS” from the holding fix.
While the AIM under 5-1-3 j.8(a) says speed reductions should be made “AT LEAST 3 minutes prior to crossing the holding fix.
Is this just a typo in the FAA documents, or am I just misreading it?
Because starting speed reductions before 3 minutes makes sense because it ensures you are at the right speeds to stay within the “protected airspace” but I’ve been taught the correct procedure is to slow down no sooner than 3 minutes before the holding fix and that is backed by the ACS.
I’m not sure which is the correct procedure or if it really matters all that much, I just have a check ride coming up and I want to be sure I do the correct procedure.

5-3-8-d in the AIM:

d. When an aircraft is 3 minutes or less from a  clearance limit and a clearance beyond the fix has not  been received, the pilot is expected to start a speed  reduction so that the aircraft will cross the fix,  initially, at or below the maximum holding airspeed

This version seems consistent with ACS guidance.
Later, 5-3-8-j-8-(a) says:

(a) Speed. When ATC furnishes advance notice of holding, start speed reduction to be at or below the maximum holding speed allowed at least 3 minutes prior to crossing the holding fix. If advance  notice by ATC is not provided, begin speed reduction  as expeditiously as practical.

The nuance here is that the former applies when YOU are assuming a self-imposed hold is coming because you are approaching your clearance limit.  The latter applies when your ATC has either directed you to hold, or to expect a hold.
Clarification edit:  I think the idea is that if ATC is about to give you a further clearance on a tightly sequenced leg, they don’t want you slowing down until the last possible moment.  They may have been seconds away from clearing your further, and just falling behind on comms.  But if ATC has already decided you are holding, you might as well not be in a hurry to get into the hold.


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What is the most efficient way to reduce interference drag when surface imperfections are unavoidable?
Interference drag can start to appear when there’s imperfections or excrescences on an aircraft outer surface. For example the following image shows some of these typical imperfections:

Source of image:https://aerodyn.org/idrag/
These imperfections is sometimes unavoidable due to manufacturing restrictions. Under such circumstances, what is the best way to possibly reduce this interference drag produces by the imperfections? Are the smoother-looking “waves” or “bump” a better design than “roughness” and “cavity/gap”?

Interference drag is defined as the additional drag that appears when two or more well defined geometric entities get together.
For example, wing alone has a certain drag $D_{wing}$; engine's pylon alone has a certain drag $D_{pylon}$; and engine's nacelle alone has a certain drag $D_{nacelle}$. But when these three bodies are joined together, the total drag is bigger than the sum of the single drags:
$D_{total}>D_{wing}+D_{pylon}+D_{nacelle}$.
And the difference is defined as interference drag.
Instead, the drag due to the imperfections visible in your picture is called... well, drag of surface imperfections.

Are the smoother-looking “waves” or “bump” a better design than “roughness” and “cavity/gap”?

As usual, to answer this kind of questions, the legendary Fluid-Dynamic Drag by Sighard F. Hoerner helps us.
The following pictures are taken from chapter 5 and give the additional drag due to the relevant imperfection:

As a general rule, holes and cavities give a lower drag than protuberances and waves due to the the fact that the latter interact most with the boundary layer.

Just another small question, how much does a SHARP wingtip contribute to total interference drag?

@Frank: chapter 7.4 at page 116 of the book I linked is what you're looking for.

Sorry, for my question I was focusing on the INTERFERENCE drag that the sharp wingtips would contribute to, in comparison with rounded wingtips. That page in the book was focusing on their effects on wingspan.

@Frank: interference in respect to what? Interference of the wingtip in respect to...

interference in respect to airstreams around the wingtip. Would the mixing of different airstreams around the wingtip cause interference drag?

@Frank: interference means among different phisical components, as I explained in the first part of my answer. What you want to know is the impact of the  different wingtips on the total drag and this is answered in the book I told you

Sorry for my misunderstanding, things make sense now.

As noted by other answers, many sources of interference drag are difficult to reduce or eliminate.
However, when placing a protuberance on an aircraft -- say an antenna -- we often have the ability to choose where to place it.  It is very reasonable to desire to place protuberance where it will cause the smallest increment in drag -- this amounts to minimizing the interference drag.
To do this, you want to place the protuberance where the local flow acceleration (the supervelocities) is small.  For a transport fuselage, we can look at a simple flow solution...

Here we can see that after substantial acceleration around the curves of the nose, the flow settles down to nearly constant velocity (at essentially freestream) along the cylindrical portion of the fuselage.
Where would you place antennae and other protuberances?

One way to reduce the interference drag is the "area rule". Basically, the net cross-sectional area should change as smoothly as possible - across the longitudinal axis. The area is bound to increase in the region of wing and tail, but by reducing the fuselage section in these regions, the change in area can be minimized.

At transonic speeds, a large part of drag increase is due to interference drag, and so it is no surprise that area rule is extensively used on modern transonic aircraft, like in the region of the horizontal stabilizer of this Dreamliner.

Picture source - Boeing 787

Another way of reducing interference drag is of course - to make the surface transitions smooth! One way of achieving this is "filleting", where additional surfaces are added just to smoothen these transitions. For example, filleting is extensively used at the wing mounting region of the DC-3.

Picture source - DC-3

I think the question is actually more focused on the small local imperfections of the surfaces and not on the global surface

@sophit Perhaps that's the case, and your Answer gives a great answer for that :-)

But I agree that the terminology and the picture in the question do not match. But I suppose that the question is about the picture and not the the terminology. Anyway you're answer is also good

Btw I don't understand where the area rule is used on the tailplane of the dreamliner

@sophit If you look at the mounting area of the tail, there is a depression ("dent") in the fuselage. This is to offset the increased cross sectional area due to tail; that's the area rule. Another reason for that depression is of course  for the variable incidence (trimming) mechanism.

Ah ok, I thought it was flat only because of the movement of the horizontal stabiliser.

@sophit That's probably the main reason for it and area rule is perhaps just an added benefit. Older aircraft with trimming tailplane like the Boeing-707 didn't have this feature though, so area rule in the tail region is a relatively new addition.


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Can you switch from VFR to IFR during a flight?
Is it possible for a pilot flying under VFR to switch to IFR during a flight?

Yes, a pilot can obtain an IFR clearance while airborne. There are at least three common scenarios:

Departing an uncontrolled airport. If a pilot has filed an IFR flight plan originating from an uncontrolled airport without ATC services, and weather conditions permit takeoff under VMC, the pilot can take off, and then contact ATC once airborne to receive the IFR clearance. The pilot must remain in VMC until receiving the clearance.

Filing while airborne. There are several ways to submit an IFR flight plan into the system while airborne: via data link or via radio by contacting a Flight Service Station (FSS). This works just like filing over the phone. Once processed, the pilot can contact ATC and receive the clearance. It can take a few minutes for the flight plan to become available; the controller may say something like "on request;" the pilot must remain in VMC until receiving the clearance.

Pop-up clearance. It is possible for ATC to grant an IFR clearance without filing a flight plan. This is possible (1) when the ATC facility controls the airspace for the entire flight: for example, if you intend to land at the airport served by the facility, such as getting an IFR clearance to fly an approach into the destination airport. (2) in emergencies. (3) workload permitting, ATC can coordinate with neighboring facilities to arrange a clearance, and even put in a normal flight plan. But they don't like to do this and may refuse.

In all three cases, the pilot must remain in VMC and abide by VFR rules until the clearance is received (which may take some time). Once the clearance is received, the flight proceeds under IFR just as if it had taken off IFR.

Comments are not for extended discussion; this conversation has been moved to chat.


text:
Can you make flight time not be compensation by either not logging the time or somehow logging it differently?
If I understand, there are cases where flight time is considered compensation, affecting the legality of operations by private pilots. Can this be avoided by either not logging the time or somehow logging it differently (i.e. with a note saying "not to be counted for licensing purposes")?

are you planning to commit fraud or something?

@Federico no, I'm not even a pilot. I just saw that in some cases a private pilot can't fly in a particular scenario because the flight time is considered compensation, so I was wondering if there's some way to not benefit from the flight time to avoid that.

@Federico, the wording of the question suggests the exact opposite of what you are asking.  Someone looking to pad their logbook with fraudulent entries would ask the question from a completely different angle.

@757toga yes, that is what I am asking. I'm not a pilot; is the FAA likely to still respond if I ask them?

Thank you @757toga ! I found an interpretation that covers my question, and I posted an answer.

Yes, this can be done by simply not logging the time.
The FAA has given an official interpretation regarding this issue, the "Lincoln-Soaring Society of America 1990 Legal Interpretation."
In this case, a private pilot wanted to tow banners and be reimbursed for the expenses of the flight. The interpretation says (to summarize) that doing so would ordinarily be prohibited because the flight time is compensation, but "[o]ne solution to this problem would be for private tow plane pilots not to log their time."
Thank you to @757toga for suggesting that I check FAA interpretations.

Good find!  I had been drafting a response in my head, thanks for saving my typing by providing a better answer than I would have.

I'm not sure what to make of the FAA actively discouraging pilots from logging flight hours. What a messed up system of representation of pilots' skill and competency.

@KennSebesta it doesn't make much sense to me either. "Flight time is compensation" seems to create many more questions than it answers.


text:
Why are stealth aircraft not sky blue?
Why are stealth aircraft not painted sky blue on the bottom to make them hard to see by looking up from the ground?
At night or with clouds between the plane and ground, the plane won't be visible anyway, so color doesn't matter then. On a clear day, however, wouldn't it be harder to see the plane if it matched the color of the sky?

Because they're not trying to be stealthy in visible wavelengths... They fly too high for that to be much of a worry

see visuals (in both Q and A's) at Why does Unity look transparent? I still don't understand why they downvoted it @@ It seems that dark is better at high altitudes, "sky blue" at lower altitudes, thus those "flying lightbulb" experiments.

Are you certain there aren't any?

@ScottishTapWater I dunno, optical sensors are a bigger thing than they used to be. Although I'd be willing to bet that "sky blue" is not going to fool a modern optical sensor, I'm reminded of that Tesla that smashed into the side of a semi because it was white and incidentally matched the color of the horizon and the optical sensor couldn't tell the difference.

Aye of course, but the thing with optical sensors is that they can't deal with clouds either and require some pretty complicated optics to work well. They exist, sure, but they're generally used for secondary tracking more than anything.

@monguin well I for one haven't seen any... :-)

Anecdotally, my father was an intelligence officer/instructor for the US military for more than fifty years. He told me that the F-117 was black because a high-ranking Air Force general (4 stars) said he, the general, wanted it black because black was "meaner".  No operational reasons, just wanted a "manly-man" aircraft.

Because they are rarely tracked by eyeball. Any eyeball guided missile is short range enough to be defeated by altitude. Or if they are low enough, they aren't around long enough for the eye to acquire and track.
Instead, radar. Which does not care about paint color.
But the F-22 and -35 are a pretty much neutral/camo grey. I'd strongly wager that multiple color schemes were tried and tested before coming up with the current paintjobs.
And some are, sort of.
The paint for the F-35 65th Aggressor Squadron at Nellis resembles the Russian and Chinese entries. And they are a little bit 'bluish' in places.
https://www.overtdefense.com/2022/06/24/u-s-air-force-new-aggressor-paint-scheme-wont-impact-f-35s-stealth/

Paint can have a very real effect on radar reflectivity. Special paints are therefore used for stealthy aircraft. This was especially a problem for the B-2 and F-117 which were found to have their stealthiness decreased significantly when operating in wet environments where the paint would degrade under influence of rain. That hadn't been observed when testing the aircraft and paint as that had happened in the bone dry environment of the California and Nevada deserts!

True but enemies need multiple detection methods to pin down a stealth aircraft. One spot of a Stealth aircraft by a military observer can aid the other systems.

@jwenting - The paint formulation and application can absolutely have an effect on radar reflectivity. Color, not so much. They even have a robotic arm to apply the paint, to ensure totally even layer.

@WPNSGuy the specific pigments can have an effect, thus the colour.

I suspect that the paint color for military aircraft would often be more useful to camouflage it while it is on the ground rather than in the air.

@jwenting The pigments have an effect just like the other stuff in the paint, but not because of their color.  Color is not a property that effects radar absorption or reflectivity.

@CarlKevinson colour isn't directly, but pigment determines colour so indirectly it does have an effect :)

@BlackThorn Interesting idea.  Does the US military (I know there are others) park airplanes in places where hostile forces could get at them?  It seems like if an attacker is close enough that they could visually identify planes on the ground, something has already gone drastically wrong.

In addition to this great answer, the color of the aircraft can affect its heat signature, which is the amount of heat it emits. A sky blue color would reflect more sunlight and heat, which could make the aircraft easier to detect using infrared sensors, thus make it more unstealthy.

@Skye-AT the increased emissivity of black against a cold sky could have a similar effect - thermal emission signatures are hard to manage (and carbon black paint is conductive to a variable extent, which can be good if done just right; in fact graphite loaded materials were some of the first radar-absorbing coatings)

@nasch There is a long history of camouflaging airfields in wartime. Not all nations have an ocean between them and their adversaries. And certainly not all military bases.

@BlackThorn Yes, history.  What about currently?  And I'm aware there are other countries with air forces, which is why I mentioned that in my comment and said I'm thinking of the US specifically.

There's a lot of answers and comments here about heat signatures, radar reflectivity, etc.  What is not commonly known is that the heat of the aircraft skin is not commonly used for the initial detection of a low-radar-observable aircraft from the ground.  Radiation shadowing is used, and this is completely unaffected by any color you chose to paint the airplane.  Cosmic radiation is constantly entering the atmosphere, day and night.  A passive search sensor looks for a tiny dot in the sky that is silhouetted against this radiation backdrop.  Used reliably in many systems for decades.

The sky in most of the world, most of the time, isn't blue. It's various shades of grey. A blue aircraft would stick out like a sore thumb against grey skies. This was found out already during WW2 when your thinking was applied by several nations to paint the bellies of their aircraft. Most of them rather quickly switched to various shades of grey (though possibly economics and availability of blue dyes may have been a factor as well) :)
About the only ones who didn't change to grey during WW2 were the US Navy and Marines, they continued painting their aircraft (dark) blue all over into the Korean war, probably because it might make them harder to spot against the blue water of the sea (which often isn't blue either, but that's another story) as well as because it was tradition by then to paint them blue.
But even they switched to grey by the time of the Vietnam war, when the Air Force was still using woodland camouflage on their aircraft.
The British tried various shades of pink and yellow during both WW2 and Desert Shield/Storm to make detection of low flying aircraft against a desert background harder, but they quickly came back from that (probably in part because repainting aircraft each time they are sent to a different theater is expensive and time consuming).

Even when the sky is blue, it can still have quite a range of different hues and brightnesses, and a mismatch will again stand out. A grey plane meanwhile can be readily confused for a wisp of cloud.

@leftaroundabout that too. And there's weird things going on all over. Like the RAF discovered that glossy black is actually MORE visible in the UK skies than the orange and red they'd been using before for their training aircraft.

Wasn't there also an experiment to disguise the bottom of some prop-era aircraft by covering it with lights, making it as bright as the sky above it?

@jeffronicus that'd be https://en.wikipedia.org/wiki/Yehudi_lights and results were mixed

Coloring of military planes changes, because, and when, typical strategy of use changes.
The first wide used stealth plane, F-117 was colored just black, because it was built only for top secret night missions, and the same considerations chose the color of the B-2.
But other military planes are used in other conditions, or have other considerations, so other schemes were chosen.
Examples:

During WWII, planes were mostly colored green, because they were mostly worried about camouflaging them while on the ground, and it was not considered significant, how far they could been detected in flight;

When, after two world wars, it just so happened that the world pursued for speed, and due to the advent of the rocket/space race, most planes were just polished and covered by a transparent varnish, because it was considered that, their time ending (will lose to rockets and to lasers), and they will be detected by radars, and polished surface considered better for speed;

Then a lot of camouflage schemes were used, each country created its own, some schemes even are different on the top and bottom (Ukrainian planes on top are camouflaged to hide when looking on them with the ground as a background, and on bottom, they are sky blue);

Now, the newest B-21 is shown in light gray, and this fact could tell us that it is considered to be used on typical cloudy day.

part of the reason for the black paint on the F-117 and B-2 (apart from cosmetic and mission) was that the special radar absorbent pigments used in their paints were black.

@jwenting, may be You right. I seen b/w photos of "dirty" U-2, covered with absorbent coating, I can't remember any suggestions about it's real color.
But I also know from chemistry, it is relatively easy, to make multilayered coating, and this is not something special, it is used at least from 1990th, example automobiles with color named "wet asphalt".

adding more layers increases mass, which is something you don't do with aircraft unless you absolutely have to

Per the B-21, I'm surprised more supersonic aircraft aren't painted white or a shade of light gray to reflect more visible and IR light. Surface heat due to drag is inevitable. So using a dark color will only cause the external surfaces to heat up even faster.

@DavidKryzaniak
This is not the case now. Approx in 2000s invented pigments, which look dark, but reflect IR, near like white.
These pigments already used even in office buildings, so they are not prohibitively expensive for military.

One other thing to keep in mind is that the bottom of an aircraft is by definition in the "shade" most of the time, that is, the side facing the most away from the sun during most of the daylight hours.  As such, looking up at the sky the aircraft will look darker no matter what color it is due to essentially being "lit from behind".  Thus, painting a color that matches the sky would essentially be self-defeating because you would see something darker than the sky itself.  If you want to minimize the chances of being seen from below the best color might be a matte white, so as to appear as light as possible from underneath, while avoiding reflected flares which would make the aircraft visible from further than usual.  This will still not work if the sun is directly or close to directly above (behind) the aircraft as it will still appear almost black due to the extreme contrast in lighting between the light source and the underside of the aircraft.
Of course, as other answers indicate, the aircraft color often makes little difference because of the threat model, e.g. flying too high or too fast for it to make a difference if somebody visually spots it.  For slower moving aircraft that may linger it may make sense.  Some aircraft such as police helicopters often have white on the bottom, although I doubt it makes much difference.

I have to say that the newer. Last maybe 5-6 years the flight check aircraft have been painted sky blue top and bottom. As I understand it they were used to check flight procedures in Afghanistan and maybe Iraq I believe. One day when they were newly painted I over heard a pilot ask the controller what they thought of the new paint scheme. The controller responded that it was harder to see. To pilots response what that’s what they were hoping he would say.

That was a very specific situation, with a very specific colour of the sky. Had they flown the same aircraft with the same paint scheme against a dull grey British sky the effect would have been quite different.


text:
Is there any altitude below which an unlicensed pilot can fly over private property?
First: this is a hypothetical question; I am not going to fly a plane anywhere at any altitude unless/until I am properly trained and licensed.
Suppose I am not a licensed pilot, but I own a large area of land and a Cessna 172. Is there any altitude below which I could legally fly the plane over my own land?

If you like rules and restrictions then aviation is for you.

No; you cannot legally cause an aircraft to “move under its own power for the purpose of flight” (14 CFR 1.1, Flight Time) without some sort of license unless it falls under an exemption, which your Cessna 172 does not.
Arguably, you would be breaking the law as soon as you started the engine, before you even left the ground.
However, there is a huge exemption for ultralight aircraft.  The idea is that they’re so small and slow that you’re unlikely to do serious damage or injury to anyone other than yourself.  (The FAA exists to protect the general public, not pilots.)  But you’d still be wise to get some training for your own sake.

Comments are not for extended discussion; this conversation has been moved to chat.

The correct answer is that federal regulations prohibit an unlicensed pilot from flying a certified aircraft in US airspace under FAA jurisdiction.  Given that Class G airspace extends to the surface, that means pretty much anywhere in CONUS.
However, there are a few things worth considering that might make such an act defensible...
Consider for a minute that if you are reading this in the United States then as soon as you step outside your abode you probably protrude into Class G airspace.  Class G is uncontrolled by ATC of course, but otherwise does the FAA exercise any authority over you for the fact that you are now in airspace that they have jurisdiction over?  Of course not, they only care about the airspace airplanes commonly operate in, i.e. "navigable" airspace of the National Airspace System (NAS), not the private, personal airspace just above your backyard.  But where is the altitude cutoff that delineates private airspace from the NAS?
I couldn't find a consistent answer to this, but there are other regulations and legal precedents that can help us understand a bit more about what is considered navigable airspace for the purpose of FAA jurisdiction.
This article provides a useful perspective.  It states:

Before the advent of air travel, landowners owned an infinitely tall
column of air rising above their plot. (The Latin doctrine was Cujus
est solum ejus usque ad coelum, or “whose is the soil, his it is up to
the sky.”) In 1946 the Supreme Court acknowledged that the air had
become a “public highway,” but a landowner still had dominion over “at
least as much of the space above the ground as he can occupy or use in
connection with the land.” In that case the court held that a plane
flying just 83 feet in the air—the commotion was literally scaring the
plaintiff’s chickens to death—represented an invasion of property. The
justices declined to precisely define the height at which ownership
rights end.

Apparently drawing a parallel with the altitude restriction in uncongested areas, it goes on to say:

Today, the federal government considers the area above 500 feet to be
navigable airspace in uncongested areas. While the Supreme Court
hasn’t explicitly accepted that as the upper limit of property
ownership, it’s a useful guideline in trespass cases. Therefore,
unless you own some very tall buildings, your private airspace
probably ends somewhere between 80 and 500 feet above the ground.

Another useful perspective can be gleaned from the FAA restriction on recreational drone flying, whose maximum altitude is capped at 400’ AGL to deconflict with aircraft operating within the NAS.  There may be additional examples of regulations on antenna heights, max height for flying kites, etc, that can give a more complete picture, but I will stop there.
While it would be unwise to challenge this, a skilled lawyer arguing before a sympathetic jury could probably keep you out of hot water if you were to fly at 400' AGL or below in the private airspace over your own personal land.

One should probably add that "altitude is your friend" in terms of flight safety; a new pilot staying below 500 feet AGL is all sorts of bad idea -- and one who had his license pulled likely wouldn't stand up well in court...

@ZeissIkon, totally agree on the safety of flight aspect, I infer that it would be a bad idea in a couple places already.  Kind of thought it was a no brainer.  Regarding enforcement actions, there is no license to pull...

I was referring to a formerly licensed, as opposed to never-licensed individual.


text:
How common is QRM on air band radio?
How common is it for QRM to interfere with aviation radio, whether from multiple operators transmitting simultaneously, from an unlicensed transmitter, or from sources other than air band radios?

In my experience it is primarily “doubling up” where two stations start transmitting at once and step on each other.  This happens quite regularly, in my area it would certainly be a daily occurrence.  It’s generally not a problem.  One of the advantages of AM is that it’s easy for a listening station to tell when two stations have talked simultaneously, whereas the “capture effect” of FM detection makes that harder to discern.
In 30+ years I have very rarely ever encountered other RF interference, (maybe once or twice?) and have never encountered nuisance transmissions.


text:
How do rental prices that include fuel work if you refuel?
If a plane rental price includes fuel, what happens if you land and refuel during the trip?

Every FBO that rents aircraft wet should have a published policy on this, possibly even in the rental agreement you signed.
In every case I’ve seen or heard of, you just submit the receipt for any fuel purchased (showing the tail number and date/time) and they’ll credit that amount toward what you owe for the rental.
Note that they may have a maximum price per gallon, typically what they’re currently selling gas at, so it’s worth checking that so you can avoid paying more than that when you refuel elsewhere.
Since there’s no reward for paying less than their maximum, though, there’s no reason to go out of your way for the cheapest possible fuel as you might if you’re an owner or renting dry.

I have encountered at least a few FBOs that credited based on gallons_purchased x local_price, so the renter ate the cost difference for buying fuel out at a higher price, but if they found cheaper fuel it was effectively a discount on the rental.

When I've rented before (admittedly many years ago), the FBO / flight school provided a (credit) card to use for fueling, and they paid the monthly bill of course. Receipts were submitted for verification. Charges on the card not related to (fueling) the airplane could result in you not being allowed to rent anymore.
In the event that the provided card didn't work or you had to pay using your own card for some reason, you would submit the receipt and receive credit for the amount paid (usually up to some maximum price per gallon).

It's often a "fuel card" which only is accepted by airport fuel providers, and purchases are restricted to fuel and perhaps related minor maintenance like oil and tires.


text:
Do you have to look outside at all under IFR?
Do pilots flying under IFR ever have to look outside the plane?

The idea of not looking out the plane when flying seems just... weird.

@Michael it seems very weird to me too, but pilots can fly in zero visibility conditions, which also seems weird, and, at least to a non-pilot, seems equivalent to flying with cardboard over the windows.

One flys IFR for reasons other than just visibility.

@Michael unless you are expecting nuclear detonations and have the blackout curtains pulled.

@Harper-ReinstateMonica how would curtains do any good again nuclear blasts? Are they made of lead?

@someone For nuclear blasts far enough away not to crash you with shockwave effects, they prevent you from being rendered blind by the flash.  The radiation exposure will be a negative prognostic indicator for cancer in the long term, but you'll hopefully not crash your plane, and if it's a "cargo" delivery hopefully you'll get it to the "customer" who requested it.

@Harper-ReinstateMonica oh okay, that makes sense! Thank you!

Yes.  When taking off. When landing. When not in clouds. And when in clouds looking for icing.

So then not in clouds, you do look outside just in case there's another plane where there shouldn't be one, but that's unlikely enough that it's safe to fly inside clouds?

@Someone That's not quite right. In some airspace, even if you're flying IFR, other aircraft are allowed to be flying VFR, and ATC may not tell you about them. So you have to look outside in order to avoid those aircraft (which are operating completely legally). On the other hand, VFR traffic should never fly inside clouds, so it's considered safe to be in a cloud without looking out for traffic.

In the US, pilots flying under IFR are responsible for keeping a visual lookout for VFR aircraft anywhere that those VFR aircraft are allowed to fly (except in Class C or higher airspace, in which case ATC provides separation.) In some cases (Class G airspace) this can include the airspace immediately beyond the edges of the clouds.  Obviously this is inherently a bit problematic.
Related: How does 700 AGL class E protect IFR traffic?

True that ATC provides separation between VFR and IFR traffic in B and C, but that doesn't relieve the PIC of responsibility from exercising "see and avoid" while in VMC.

The phrase from the comment "pilots can fly in zero visibility conditions, which also seems weird, and, at least to a non-pilot, seems equivalent to flying with cardboard over the windows" reminded me of Aeroflot Flight 6502 crash where the crew had a bet and actually flew with curtained windows. The outcome was 64 passengers + 5 crew dead out of 85 passenger/8 crew. I am not sure if the airport was equipped with any sort of ILS back in 1986.
Ironically, both betting pilots survived, but one later died on his way to hospital. The other one, who actually failed the attempt, was sentenced and served 6 years in prison.

More likely NDB and PAR.

Probably not PAR or they would be informed about being too high, the report says 30 m too high above the outer marker and 16 too high above the middle NDB/marker (they were called the farther one and the nearer one, always in two).

Yes. You are required to see and avoid as much as possible. So, if you're in Visual Meteorological Conditions, you need to be looking outside.

This adds nothing to what's already been posted, with citations.

In 2014, Airbus patented an aircraft design which includes a cockpit that has no windows.
I know Airbus has a lot of money, but I don't think they'd go so far as to patent an airplane that would be illegal to fly.  I could be wrong.

-1 for a few reasons: this suggests the answer is "no," which is wrong. Granting of a patent does not in any form whatsoever imply regulatory approval, expectation of such, or intent to build or market: patents are about intellectual property only. Finally, the point of Airbus's patent is that the windows are replaced by electronic display screens, so the pilots could still see outside in such an aircraft, as required by current regulations.

" I don't think they'd go so far as to patent an airplane that would be illegal to fly. I could be wrong." Yes, you are wrong in this.

Thanks for the feedback -- now I'm inspired to ask a question more-specifically about this situation.

They might have filed the patent to dead-end the concept. Now nobody can build it for 20 years without Airbus' consent, and they always say no.

authors: Vladimir F Героям слава


text:
Why do many small planes allow flight control from either side?

(From Wikimedia Commons)
Unless the plane is being used for training, what's the point of having flight controls on both sides? As far as I know, it's not common to have a copilot on planes like this (Cessna 182).

FWIW most aircraft have a procedure to remove the second set of controls when desired (like to fit cargo on that seat instead). As to why they come with dual controls by default despite being a single pilot operation well... I have a theory but good question.

@Ben what's your theory?

It's an awful lot of work to remove the second set of controls in between times it is being used for training, and for what purpose?  Just because a plane isn't dedicated 100% to training, or doesn't require a co-pilot doesn't mean two rated pilots might not want to share flight time...

Nearly all small planes are used for flight training, even if only intermittently; adding and removing the second set of controls each time is not worth the cost.

So that you can fly over Great Britain and Australia! ;-)


text:
Can you study for the written license test online?
I would like to get a PPL at some point. However, because of costs, that's likely to be years in the future, if ever. I know I'll take a course that covers what's on the written test as part of getting the license, but I would like to start studying online before then so I'll already be familiar with the topics. Are there any no-cost (and preferably open-source) websites or Android apps for studying?

I'm guessing you are in the USA?

@tedder42 yes, I am in the US.

keep in mind that your theory certificate is valid only for a limited time, you MUST retake it if you don't pass your practical test within that time. Not sure how long that interval is, but it isn't more than a few years and may be significantly less.

@jwenting I wouldn't actually be taking the test, just studying the material so I'm familiar with it when I do take the test eventually.

Probably the best place to start is the FAA's Pilot's Handbook of Aeronautical Knowledge, (PHAK).  You can find it here:  PHAK  You can also purchase paper copies on-line or at a local flight school if you prefer.
You will also need to be familiar with the Federal Air Regulations, (FARs) in 14 CFR, (Code of Federal Regulations).  In particular, parts 61 and 91 of chapter I.  I have found Cornell University's legal website most useful for searching these:  FARs
And finally there is the Aeronautical Information Manual, (AIM).  You can find PDF versions on the FAA's website, but I have always used the paper version, published annually, bundled with the FARs, and referred to as the "FAR/AIM".  They don't change much, so while you will want the latest when you are doing final study prep for the written, for early learning and high level overview you could probably get an outdated paper copy for free or very cheap if you want to mark it up and add tabs, etc.
I would start with these as the "big three".  Of course there are other resources as well, but these will get you off to a good start.

There are any number of "Online Flight Schools" that can be found by using a simple search on your favorite search site.
Some appear to offer a fixed price for the course, others offer a monthly subscription.  Some appear to offer guarantees that you'll pass the written test.
No cost?  Not really other than there are multiple resources available free from the FAA web site that have all the information that you need to know to pass the written exam.
FAR/AIM - FAA FAR/AIM
ACS - FAA ACS
As well as many others.  There are also sample exam questions:
Sample Test Login
The problem with the FAA site is that it's very easy to drown in information.  An organized course can help you focus on in the most important topics that you need to pass the exam.


text:
Changing SID without changing the flight plan
Pilots are demanding conventional SID rather than RNAV SID planned on flight plan due gps problem. In that case is it enough for a tower controller to coordinate the conventional SID with the approach unit or should pilots contact with their company to refill the flight plan?

The SID is normally assigned by the approach controller, not the scheduler, as SIDs are runway dependent and may change at a moment's notice with changing runways and because of noise abatement requirements.
A "default" SID is probably suggested by the airline's planning department based on historical patterns but there's no guarantee you'll actually get it.
If the SID you're actually getting doesn't match the rest of your flightplan, ATC will vector you. Just as ATC will give you instructions at any time they want you to do something that's not filed with them as your flightplan.
The airline has nothing to do with that, though they may want to know afterwards for their statistics and to explain possible extra fuel burn.

Just to add, for several departures in Europe there is a note on the chart to inform ATC (or specifically clearance delivery) that one is unable RNAV1 or RNAV (in general).

Is there an area-wide GPS problem or a problem with the equipment on a particular aircraft?

We had that problem with several aircrafts at Istanbul airport it was not area-wide and yes SID is runway dependent for example for runway 36 SID ivgus4e and for runway 35L it is ivgus4d.

@757toga it's the same in the US and everywhere.


text:
Low visibility in progress and low visibility in force
Are Low visibility in progress and low visibility in force different?

Where did you encounter these terms?

Atis of different airports. Some of them low visibility in progress some of them low visibility in force

We use "Low visibility procedures in operation". Same thing :)

I believe that these two terms most likely refer to the same thing, namely:
Low Visibility Procedures (LVPs) are in force at this Aerodrome.

Low visibility procedures (LVP): Specific procedures applied at an
aerodrome for the purpose of ensuring safe  operations during Category
II and III approaches and/or low visibility take-offs.
ICAO Doc 9365, Manual of All-Weather Operations

Your first statement, "low visibility is in progress", lacks clarity and objective criteria to define what constitutes "low visibility." Without specific parameters, such as visibility distance thresholds or industry standards, it is difficult to determine the extent of low visibility. Visibility can vary depending on factors like the airport's definition, airline policies, and prevailing weather conditions. To ensure clarity and precision, it is important to establish clear criteria for low visibility, such as specific visibility distances, meteorological conditions, or adherence to internationally recognised standards. This allows for a more accurate and consistent assessment of low visibility conditions at airports.
Hence why I believe that you are inquiring about the defined “Low Visibility Procedures” in which, when in place, an airport changes its operations. For example procedures for taxi operations, holding points, ground lighting etc etc. (For further info see: ICAO Doc 9476, Manual of Surface Movement Guidance and Control Systems or Skybrary Low Visibility Procedures in the context of EASA regulations

Yes, "low visibility in progress" and "low visibility in force" are different terms used to describe different stages of low visibility conditions.
"Low visibility in progress" typically refers to a situation where visibility is gradually decreasing or becoming worse. It suggests that the conditions are deteriorating and may continue to worsen.
On the other hand, "low visibility in force" indicates that low visibility conditions are already present and have reached a certain level of severity. It suggests that the conditions are currently in effect and may persist for a period of time.
In summary, "low visibility in progress" suggests a developing situation with decreasing visibility, while "low visibility in force" indicates that the low visibility conditions are already present and ongoing.

Do you have a source for this?


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How come deflected flaps cause a nose-up pitching moment in high-wing aircrafts and a nose-down pitching moment in low-wing aircraft?
I imagine that the increased downwash due to the new camber of a high-wing aircraft causes a down-force on the horizontal stabilizer which in turn causes a nose-up pitching moment. However, I read that the increased drag also causes a nose-up pitching moment in high-wing aircraft. I don't understand how though. The drag acts on the center of pressure but how does that cause a nose-up pitching moment in a high-wing aircraft. It's assumed that the center of pressure lies aft of the CoG. When the center of pressure moves further aft due to flap deflection, then how can increased drag cause a nose-up pitching moment? The lever arm w.r.t. CoG grows and would rather cause a stronger nose-down pitching moment.

"How come deflected flaps cause a nose-up pitching moment in high-wing aircrafts" - I remember a pronounced pitch-down change in attitude with full flaps in the C152 and C172, but that was a long, long time ago.  Memory may be playing tricks on me.

@WayneConrad at least in the C150 I flew, adding flaps created a nose-up pitch moment that had to be counteracted with nose-down elevator. The flaps-down attitude ended up more nose down than with flaps up.

@CameronSS That's makes sense, thanks for the explanation.  I never flew those planes, but was a passenger, so likely only the end result was really noticeable to me, with the initial pitch-up moment being compensated for by the pilot without me knowing it.  But I distinctly recall how much it seemed like I was looking straight down after flaps were applied in those planes.

The center of gravity is a point in three dimensions, not simply "aft or forward" but also located somewhere "up or down." Since drag is opposite to the aircraft's motion, the relevant lever arm is not how forward or aft the center of gravity is, but how far above or below it is from the center of pressure.
If the center of pressure on the wing does not correspond to the center of gravity, then drag produces a torque around the center of mass, corresponding to a nose-down pitching moment if it is below the center of gravity, and a nose-up pitching moment if it is above.
In a typical high-wing airplane, the wings are above the center of gravity, and so drag on the wings causes a pitch up moment, which is increased when flaps are deployed. The opposite is true for low-wing airplanes.

Lowering flaps should cause a pitch down moment of the wing, but a secondary effect of downwash on the horizontal tailplane can affect the actual pitching motion of the aircraft.
This is markedly noticeable on the high wing Cessna 172 when flaps are deployed, resulting in a pitch up motion.
With low wing aircraft, especially with a T-tail, lowering flaps will not have as great an effect on the tail.  In this case, only the pitching moment of the wing is affected, resulting in a pitch down tendency.

Typically with flaps on any kind of wing, they change the camber of the wing when deployed, causing the center of lift (CL) the shift forward on the chord line, resulting in a marked pitch up.  This requires the pilot to trim the aircraft nose down in order to compensate for this. Conversely, when flaps are retracted, the center of lift shifts aft on the chord line, requiring nose up elevator trim to compensate.  This phenomenon does not change based on the position of the wing in regards to the fuselage, but can be a little different, depending on the type of wing flaps used.

Shouldn't a backward shift of lift cause a pitchdown moment?

Assuming that we're talking about trailing edge flaps, why would deploying them cause the CL/CP to move forward? Since the camber is being increased at the aft of the wing, shouldn't the CP move aft?


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glide ratio of an airplane formula
Good morning everyone, I found out that the glide ratio of an airplane can be calculated by this formula : 1/tan(y) considering y the angle formed by its nose and an horizontal line.
Check this photo :

Imagine if y = 0. That means that we would divide by 0 in our formula. That's what I don't understand.
Thanks for answering by advance !

Is "y" the red or the green line?  Normally we'd consider the trajectory (green), not the body angle (red), when thinking about glide ratio, because we want to know how much ground we can cover for the altitude we have. So a body angle of zero (horizontal) wouldn't matter so much if the trajectory is X degrees down.  On the other hand, if the trajectory itself is horizontal, you don't really have a glide ratio.  As that trajectory approaches horizontal, the glide ratio gets bigger & bigger; if your flight path is horizontal, then "glide ratio" is meaningless & undefinable.

@Aditya Sharma answer is correct.  However, no reason is given for why $\gamma$ will never be zero or negative for a gliding aircraft.
The reason relates to the causal relationship between L/D and the flight path angle.  Thinking that a zero flight path angle would lead to a divide by zero and therefore infinite L/D is not the path of causality...
The aircraft has a finite L/D that it is operating at.  That leads to a certain flight path angle that can never be zero (or negative).  I.e. L/D causes FPA -- not FPA causes L/D.
Experimentally, we might choose to estimate the L/D by observing FPA in a glide -- but that does not mean causality follows that path.

$\gamma$ is the glide angle, not the aircraft pitch angle. Other than that, the formula is correct.
For the purposes of determining the gliding capabilities of an aircraft, it is assumed that there is no vertical motion in the freestream air (which would otherwise alter the glide angle). Under such conditions, $\gamma$ will always be a positive angle - it will never be zero or negative.
In practice, there will always be some vertical motion in the freestream air, which will improve or worsen the glide angle. Specifically with strong up-currents, it is possible for the glide angle to be zero (level flight) or even negative (climb). In such cases, the aircraft would be able to stay airborne indefinitely, so using the term 'glide ratio' would be meaningless.


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How to prevent altitude loss or keep flight path angle constant while doing aileron maneuver?
I am trying to perform an aileron roll maneuver for an un-manned aircraft in a simulation. The lateral control architecture is that I am controlling roll rate (stability) with aileron stick and side-slip angle with rudder stick, in which there is also a yaw damper in the inner loop. The longitudinal controllers are speed, and pitch rate is being controlled by the elevator stick. While doing the aileron roll maneuver, my pitch attitude gets affected significantly and the aircraft losses a lot of altitude. I need to perform this maneuver while maintaining the almost same altitude and heading? Can anyone advise me what am I missing in my lateral and longitudinal controller architectures?

Probably a better fit on Drones.SE.

Its related to the fighter aircrafts, so I posted here.

An aileron roll is done semi-ballistically, that is, the roll is done while the aircraft is pitched up slightly, the rolled at the maximum rate with elevators neutral, and being completed as the airplane has completed a very shallow ballistic arc.
Therefore, it has to start with ample speed and an initial pitch up, then the pitch is mostly unloaded as full aileron is applied. During the time the airplane is rolling, the nose will fall through in a more or less ballistic arc, returning to wings level by the time the nose has gone from 10-20 deg nose up to 10-20 deg nose down. Try this:

Lateral control should be maximum roll rate.

After adequate speed is attained, pitch control should be mild pitch up until the nose is 10+ deg above
the horizon, then pitch neutral as maximum aileron is applied. Just
let the nose fall through. At this point your are a spinning ballistic object, like an arrow. Rolling at maximum roll rate should
complete the roll before the nose has fallen though too far. If the roll is completed with the nose pointing down 45 deg, the entry speed was too low, the pitch attitude at entry was too low, or the roll rate is too slow.

Yaw control should just be counteract adverse yaw, to the extent
there is any.

An aileron roll is not the same as a slow roll, which transfers lift from the wings, to the fuselage, to the wings upside down, to the other side of the fuselage and back to the wings, and which requires complex elevator and rudder control inputs to achieve those conditions.

It doesn’t have to change altitude like a spinning arrow. Constant altitude may be maintained. The pitch input should be nose up as the roll begins and then shifts all the way to pitch down while inverted and back to neutral by the completion.

You are arcing a bit. You aren't making more than minor inputs to hold the nose up while knife edge or upside down. If you are applying rudder to hold alt while sideways, and pushing forward to hold it while upside down, technically you are doing a slow roll.

why would that be considered a slow roll when the rate of roll is quite fast? The inputs for wings inverted would be maintaining -1G. That would be transitory, obviously, but it would keep the nose at a 0 fpm attitude.

Those are slow roll inputs even if you are speeding up the roll rate.  You don't go 1 G negative doing an aileron roll; you'll be slightly positive, 0 or slightly negative. And you don't feed in rudder to make the fuselage make lift while knife edge. You don't need to when the maneuver is completed in 1-3 seconds. Nose up, crank it around, and a slight ballistic arc results as you finish with the nose slightly down.

pitch attitude is affected and the aircraft loses altitude

Losing altitude in a roll maneuver will happen unless enough lift can be generated with the fuselage angle of attack and the vertical component of the thrust vector.  Highly overpowered and lightweight models are capable of this.  Rudder input is required in the quarter roll.
Change of pitch is the result of the aircraft being rolled sideways. Natural (longitudinal) stability will cause the nose to dip as it descends.  Again, rudder input is required.  When the plane is sideways, the rudder becomes the "elevator" (of the nose).
To do all this, it might be better not to have a yaw damper function.
Also, the drone design should be as simple as possible.  Work on the inputs for smooth rolling manually first.  Then it might be possible to program them.
One approach can be to avoid excessive static stability.  Move the weight back, if you can, to nuetral.  This will help keep the nose up in the roll.  In my early days, I built a sport model that was actually a bit tail heavy.  Very difficult to control, but one time, with full aileron, it rolled 2x before I even knew what I saw.

You are describing a slow or 4 point roll, not an aileron roll.

Well, ailerons could be made more up than down to minimize adverse yaw, and roll rate will depend on scale to some degree.  Rudder is still helpful. What amazed me about the sport plane was it rolled perfectly with no change in pitch at approach speed.  I was dumbstruck.

If you apply rudder in sympathy with the aileron, that is more than needed for adverse yaw, in other words help the roll with skid, you will certainly increase the roll rate. All you have to do then is yank the stick back as you do so and you are in a snap roll. On the other hand, if you're using rudder to keep the nose from dropping, you'll be feeding in opposite rudder, peaking the input at knife edge, then taking it out and feeding it forward elevator as you go inverted, and so on.


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Can I get a 1st class medical after having one seizure?
11 years ago(when I was 8 years old) I had an unprovoked seizure. I have been off meds for almost 10 years now. I never had another seizure. Since that night all EEG’s I had done have detected 0 seizure activity and I had an MRI done that showed nothing wrong with my brain. Is it possible for me to get my 1st class medical or special issuance?

Were you diagnosed with epilepsy? If not, was a cause of the seizure determined? If a cause was determined, and it was not epilepsy, you may not even need an SI. https://www.faa.gov/faq/what-medical-conditions-does-faa-consider-disqualifying-0

Nope no epilepsy at all. A cause wasn’t 100% determined that’s the issue here. The doctors were pretty certain that it was caused by heat exhaustion and dehydration because it was the middle of July

The FAA might consider that a "satisfactory explanation of cause"; I don't know what their standard for a "satisfactory" explanation is. If they don't consider that satisfactory, a special issuance may still be an option.

Yea I really hope I can get it either route. I would hate to have my dreams crushed because of something that Happened so long ago and that I’m certain will never happen again. Do you think special issuance is still possible?

I'm not an expert on aviation medicine, so I'm not sure. It does sound like a possibility, but someone more knowledgeable than me should be able to give more information.

Find an AME who routinely deals with tricky medical issues and schedule a consultation. DO NOT schedule your exam. He/she can tell you with more confidence what your chances are.

I’ve been emailing an ame back and forth and he thinks I should be able to get it. I have an exam with him soon.

@RetiredATC how would you go about finding an AME that routinely deals with tricky medical issues? Living in the Los Angeles area.

Pilotsofamerica dot com has a medical forum. There is a lot of information there as well as AMEs that specialize in difficult cases. I've read many instances where they were very helpful. Ask your question there and I think you will find what you need.

Nobody can say without knowing your full medical history, but it is possible if this is your only medical issue.
The FAA's guidance to AMEs on seizures can be found here. From what you've said, you would fit into row B or C (it doesn't really matter which, since they want the same information either way). The documents the FAA will want to see are listed in that document. Make sure you have all of them, and at the correct time periods before your AME exam is scheduled.
Seizures are also mentioned in this Federal Air Surgeon's Medical Bulletin. It mentions that they have a 3rd class medical certificate to an airman with a history of six seizures, so a single seizure is definitely not a deal-breaker.
You are very likely to be deferred, and to may need to undergo additional monitoring for a period of time. But if all the required tests look good you should be able to get a special issuance.

I am very very healthy. I have 0 other medical conditions. I take no medication. I’m pretty sure the ame has to defer me. I don’t believe he can issue me with the seizure. Do you think I can only get a special issuance medical?

@ryguy_004 I am also pretty sure that the AME has to defer you. The FAA can decide to issue you a normal medical or an SI. Note that an SI is just as good as a normal medical, you just need to send the FAA some medical information every once in a while to keep them happy. The FAA may decide you no longer need an SI and issue you a normal medical at some point in the future.

Yea I really hope I can get either. I honestly don’t know what my chances are.


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Why cut off the tips of a delta wing to form a cropped delta?
According to wikipedia, the reason to cut off the tips of a standard delta wing is

maintain lift outboard and reduce wingtip flow separation (stalling)
at high angles of attack

This type of delta wing is named cropped delta.

I don't quite understand this sentence. What does it mean by "maintain lift outboard"? And does this modification prevent stalling?

It may be called a "cropped delta" but that is not the actual genesis of this design.  The very shallow chord at the point of the wingtip is producing very little lift.  "Cropped" makes it sound like they started with a 40 foot wingspan design and just cut two feet off each wingtip creating a 36 foot cropped delta. (Image on left)
But you can look at that from a different perspective.  That same picture could be an aircraft designed to have a 36 foot wingspan, but the chord length of the wings were lengthened (creating a deeper wing front-to back) to maximize lift. (Image on right)

In other words your picture is not a 40 foot delta with the wing tips clipped, it's a 36 foot delta with a few feet of additional chord added to the wing.
It doesn't prevent stalling, rather it delays stalling and improves roll control in a stall.  In the picture I posted, the yellow aileron/elevon on the left is exposed to the turbulent flow behind a tiny wing chord.  On the right, the yellow aileron/elevon doesn't start experiencing turbulent flow nearly as early.


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Does the 787 vertical stabilizer have a connection fairing?
In general, a vertical stabilizer connected with the fuselage will form junctions. The junctions trigger a mix of airstreams, and causes interference drag. A fairing is used the ease the sharp junction angle and reduce this type of drag. Like between the junctions of the aircraft's main wings and the fuselage.
On an A320, fairings are also installed between the junctions of the fuselage and the vertical stabilizer.

However, on an B787 aircraft, it is not easy to discover the fairing installed for the vertical stabilizer. Does the 787 really have this fairing? If not, what stops the manufacturer from adding this component, and reducing drag?
Does the 787 have this fairing?  Image source:https://www.alamy.com/stock-photo/boeing-787.html?sortBy=relevant

It does not necessarily reduce drag.  One fuselage design goes through CFD and wind tunnel testing, and reveals a pressure delta that justifies a fairing.  Another slightly different fuselage design also goes through CFD and wind tunnel testing, which reveals there isn't a need.  In complex design, the answer to "why didn't they..." is almost always because "it turned out in this case they didn't need to," and very rarely "because the wanted to but couldn't."

Fairings generally aren't needed if two surfaces meet at a 90 degree angle or more, because the interference drag from the two flow fields getting in each other's way is not significant unless the angle is under 90 degrees.  This is why you won't see root fairings on WW2 fighters with 90 degree or greater main wing to fuselage intersection angles, like the F-4U Corsair.
Looking carefully at the Airbus in the picture, it looks as if the fairing contours at the base of the fin are to enclose a structure or components at the base of the fin that are slightly wider than the airfoil contour at the base. Like fin attachment lugs.
The fairing is not a "fillet", where a radius joins the two surfaces with a smooth curving transition between the surfaces. It's more of a flaring out of the fin at the base just to enclose bits that stick out.
Most airliners, like the 787 in the second picture, are able to contain all of the attaching structure within the fin's "mold line" (the airfoil contour surface) and all you have is a straight surface meeting a curved surface of the fuselage, such that interference drag isn't a problem and the permanent sharp skin joint does the job.


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Does the cross-sectional shape of a fuselage affect drag?
Comparing two  bodies with the same streamlined longitudinal shape, and the same cross-sectional area, one with a circular cross-sectional shape, the other with a quadratic cross-sectional shape, which one of these two bodies would create more aerodynamic drag?
My prediction would be that it is the one with quadratic cross-sectional shape. As around the sharp edges where the two straight surfaces meet, flow interference would occur, which increases the total aerodynamic drag.
Note that these two bodies have 0 angle of attack relative to the incoming flow.
Related question: Why is the fuselage on an airliner circular-shaped?

Related: https://arc.aiaa.org/doi/pdf/10.2514/1.C036032    and https://apps.dtic.mil/sti/pdfs/ADA389409.pdf

The quadratic body has more surface area compared to the round body, so this alone will guarantee a higher friction drag.
If the angles of attack and sideslip were always zero and no lift is created, the edges would not cause trouble. But as soon as a lift-producing wing is attached, the interference will create crossflow which will result in a higher drag of the rectangular fuselage.

@MichaelHall Yes, a circle has the minimum perimeter for a given area.  Any non-circular shape (with the same cross sectional area) will therefore have more wetted area than a circular shape.

I had a momentary doubt, but mathed it out to confirm for myself.  Thanks!

Lengthwise, I was surprised to learn that the optimum fineness ratio was surprisingly stubby; 3:1 or something? The Questair Venture, the stubby little high perf kit plane, was close to egg shaped and it is supposed to have close to the ideal min drag profile.

Hi. Can you please explain how the "crossflow" would increase drag? I'm having a hard time visualizing it, thanks.

@Frank With sideslip the flow has a sideways component that must now negotiate the sharp corners of the rectangular fuselage. This will lead to separated flow on the leeward side and lower local pressure, which has a rearward component, adding to drag.

@PeterKämpf  I understand how this may happen in a sideslip. But in the answer you said the attachment of a lift-producing wing will cause interference that creates crossflow and add to drag. Does wing-fuselage interference have association with crossflow and drag? If so, how?

@Frank The wing produces upwash ahead and downwash aft of it which locally acts much like the crossflow in a sideslip. At zero angle of attack this does not cause much drag, but when the fuselage is inclined, this adds a bit of drag, too.

@PeterKämpf  Oh, so upwash and downwash don’t simply end at the wingroot, but extend a bit inboard, and have some effect on the fuselage. Is that correct? I believe the additional drag by this effect would be fairly small then.

@Frank correct.


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Why will all air slow down by the same amount in an adverse pressure gradient?
Boundary layer separation is caused by air slowing down to zero and reversing before reaching the end of the body. This is due to friction (viscous effects) + adverse pressure gradient.
The formation of an adverse pressure gradient can be explained by the geometry of the body. In the case of an airfoil, for example, the way I see this is that after air has been accelerated around the leading edge (strong curvature), it flows to the rear part of the airfoil (less curvature), and experiences a "collision" with the straighter surface. That collision causes a rise in pressure - an adverse pressure gradient.
Air inside and outside the boundary layer is slowed down by the same amount because of this pressure gradient. But I don't understand why. Air already slowed down by friction should "collide" less strongly with the surface, which implies a less intense adverse pressure gradient. Why will all air, regardless of some being decelerated by friction, experience an adverse pressure gradient of the same intensity?
There might be some misconceptions in my understanding, so I would appreciate any correction and assistance!

related: https://aviation.stackexchange.com/questions/58794/what-causes-the-adverse-pressure-gradient-that-leads-to-flow-separation/58795#58795

One of the assumptions of a thin boundary layer is that the pressure across the boundary layer is constant.  I.e. the adverse pressure gradient is 'created' in the inviscid flow outside the boundary layer.  It is then transmitted through the boundary layer and presses against the body.

Does this answer your question? What causes the adverse pressure gradient that leads to flow separation?

Remember the wing is hitting the air.  Surface friction interacts with air to create the boundary layer.  The "adverse gradient" (from air trying to fill the void) only becomes significant when proper circulation breaks down due to excessive AoA.

You write that air "experiences a collision" with the airfoil surface. That is a bit harsh - while air molecules collide among themselves all the time, few do it with the surface. The sum of those collisions can be interpreted as pressure: The more numerous and stronger the collisions between gas molecules are, the higher the pressure in the gas is.
High curvature produces suction in order to make the molecules change their flight path. Suction is the lack of pressure: While air further away from the suction area maintains the number and intensity of collisions, air close to the curved surface experiences fewer and less intense collisions. Consequently, fewer collisions happen with the surface. We measure lower pressure and the wing experiences lift.
Less curvature requires proportionally less suction until a straight contour will return pressure to its ambient value. A concave contour needs to increase pressure in order to push the flow along its path. In potential flow pressure is proportional to the curvature of a local streamline. Since curvature radii increase with more distance from the airfoil contour, pressure variations diminish equally. In other words: There is a (weak) pressure gradient along lines perpendicular to the airfoil contour.
So far, all this was true for inviscid flow. Viscous flow differs only in a small volume covering the surface and from a thickening of this volume with flow length, which in turn changes the streamlines around the airplane slightly. But the pressure effects are unaffected by this: What we measure as pressure are the collision components perpendicular to a surface. The change in flow speed from friction happens parallel to the surface. The slower flowing layer close to the surface still experiences the same collision intensity as the layers at a larger distance from the airplane surface. Therefore, pressure does not change in the boundary layer.

air ... experiences a "collision" with the straighter surface

Not if the airspeed is high enough.  A "healthy" circulation has the airflow, trying, but not succeeding in flowing back to the wing surface.  It continues downward aft of the rear trailing edge.
This, in simplest terms, is the "downwash" action of a given mass of air at a given velocity.  The reaction is the lifting force of the wing.
When angle of attack reaches a critical stalling point, the adverse pressure (normal atmospheric) breaks down the low pressure bubble over the wing, resulting in turbulence and loss of lift.

air inside and outside the boundary layer is slowed down by the same amount by this pressure gradient

Right.  What is observed is two effects reinforcing one another  out.  Looking at it from the "wind tunnel" point of view, the wing is stationary and the air moves.  This is also true in flight from the wing frame of reference.
So, the wing friction will slow freestream air down and the adverse pressure will tend to reverse its direction under conditions of excessive AoA.
Air tends to flow from higher to lower pressure.  With a lifting wing, the momentum of the airstream prevents the low pressure "lift bubble" from collapsing until critical angle of attack is exceeded.

Pretty much what I was thinking, but I'm afraid this is not what this question is actually about.

@Jpe61 let's hope it helps.  There are cases where turbulent flow (vortex generators) actually help delay flow separation.

For sure, but the question is about air velocity in adverse pressure gradient...

@Jpe61 yeah, we do our best not to stall, but there still is pressure  leakage at those darn wingtips. What to do?  Whale tails are looking interesting.


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Alternator vs Battery Power
What provides electricity to the electrical system of a Cessna 172? Is the alternator connected to the electrical components directly and battery is only used in the event of an alternator failure? If the battery is only used as a backup solution, why does it require a continuous charge to it? Thanks

The alternator is the primary electric source on the 172. However, the main battery does supply the power to the rest of the system (Alternator supplies battery, battery supplies to the system). Neither are directly connected to the electrical systems, but they are connected to the various electric busses that then connect to the rest of the system. The batteries are not continuously charged but have a voltage regulator that senses if the voltage in the batteries gets too low and sends power from the alternator to the battery.
Page 7-48 from the PIM for the 172S NAV III GFC 700 AFCS has a good digram for how everything is connected. Page may vary depending on what model you have a PIM for.

The voltage regulator on most Cessnas (including the 172) just adjusts the field voltage to provide a constant output, specifically 14 volts. Under all realistic conditions, the battery voltage will be less than 14 volts and so the batteries will not be supplying power because the regulator will be pushing the voltage higher than the battery can.